MINUTE STRUCTURE OF HYBRIDS. 565same form but of unequal length, those of their hybrid offspring have a lengthwhich corresponds approximately to the mean between the lengths in the parent-species. Thus the length of the hairs on the backs of the leaves is 0-3 mm. inSalix aurita, 12 mm. in SaUx repens, and 0-6 mm. in their hybrid Salix pUcata.The hairs in SaUx Caprea measure 0-8 mm., in SaUx viminaUs 0-3 mm., and inSaUx acuminata, their offspring, 0'5 mm. Whenever one stock is glabrous and theother hairy, one may be quite sure that the corresponding parts of their hybrid willbe furnished with hairs, but less profusely than the parent-species from which thatparticular characteristic is derived. This is the case, for instance, with PrimulaSturii, the hybrid produced by crossing the glabrous Primula minima with Pri¬mula viUosa, which has glandular hairs. The leaves of the latter are thicklycovered with these hairs, which vary from O'T mm. to 1 mm. in length, and PrimulaSturii has scattered glandular hairs which measure 0'3 mm. The hybrids obtainedby crossing the Purple WiUow (Salix purpurea) with the Common .Osier (SalixviminaUs) are distinguished by Botanists into two sections, one of which—Salixrubra—-approximates to the Purple Willow and the other—Salix elceagnifoUa—tothe Common Osier. The leaves of the Purple Willow when mature are glabrous atthe back, those of the Common Osier have small glistening hairs lying appressed totheir under surfaces, paraUel to the lateral nerves, and measuring 03 mm. Thereare about 1800 of these hairs on a square millimetre. The hairs of the hybridSaUx elceagnifoUa are of the same length as those of ;Si. mminalis, but there areonly about 800 of them to the square miUimetre, whilst the hairs of the hybridSaUx rubra are somewhat shorter, and there are only 400 to the square milUmetre. Recently the discovery has been made by Wettstein that the form and dis¬position of the cells and tissues in hybrids is also a combination of the correspondingcharacteristics in the parent-species. The various species of the Pine genus(Pinus) may be distinguished with certainty by the anatomical structure of theirneedle-shaped leaves, in particular by the thickness of the epidermal cells, thenumber of the stone-cells lying beneath the epidermis, and the number of the resin-ducts. In the hybrids the anatomical characters of the parents in these respectsare united, and the result is indeed often an exact arithmetic mean between thetwo. Thus a needle of the Scotch Pine (Pinus sylvestris) contains from 6 to 10resin-ducts, that of the Mountain Pine, Pinus Mughus (montana), contains from3 to 5, and that of the hybrid offspring of the two from 5 to 7 such ducts. TheJunipers (Juniperus) afford a similar instance. In their case the leaves are dis¬tinguished by the various thickness and length of the layer of sclerotic-cells whichcovers the back of each leaf, by the width of the resin-duct running through themiddle of the leaf, and by the number of the cells encasing that duct. In thehybrids, such as Juniperus Kanitzii, which is produced by crossing Juniperuscommunis and J. sahinoides, there is evidently a union of the parental attributesin the corresponding cellular structures in the leaves. It has also been shown byHildebrand that in the Wood-Sorrel (Oxalis) hybrids also the anatomical charactersof the parents are united, but by far the most comprehensive study which has